The need for countries all over the world to cut down on their greenhouse gas emissions is becoming more and more urgent. Almost every day there are news reports on these gases and the harm they are causing the world. The most interesting recent news has been that the American state of California is suing the world's major car manufacturers for damaging the state by being major contributors to greenhouse gases.

What exactly are greenhouse gases?

A simple way to understand it is to examine the name. What is a 'greenhouse'? A greenhouse is a building used to grow plants, usually vegetables and fruits, in places where temperatures are too cold for those plants to grow naturally. A greenhouse is usually made up of transparent glass, closed from all sides, so that light can enter and warm up the building. The warm air inside cannot escape. Greenhouses are always warmer inside than their surroundings. That is how they protect the plants inside from the cold outside. This is called the 'greenhouse effect': trapping heat and preventing it from getting out.

Greenhouse gases work in exactly the same way, but their effect is not so benign.

Our atmosphere -- the cocoon of gases surrounding the earth -- is made up largely of nitrogen, oxygen and carbon dioxide. But other gases also exist in smaller quantities. Some of these atmospheric gases create a 'greenhouse effect'.

When light from the sun reaches the earth's surface some of it is absorbed, warming up the earth. The heat from the warmed-up earth is reflected back out into space in the form of infrared radiation. Part of the heat going into space is absorbed by greenhouse gases in the atmosphere before they are lost to space. The gases themselves, once they have absorbed the heat, release it slowly -- some upwards into space but most downwards to the earth's surface. Kind of like a greenhouse, with the gases acting like the glass walls and roof of the greenhouse. Or, an even better metaphor is that these gases warm up the earth the way a blanket warms you up on a cold night. Greenhouse gases are the earth's blanket.

Some greenhouse gases are natural and have always been there in the atmosphere. Some, though found in nature, are also created by human activity. Others are rarely found in nature and are wholly human creations.

The major atmospheric constituents -- nitrogen and oxygen -- are not greenhouse gases because they do not absorb or emit infrared radiation. But other naturally-occurring atmospheric greenhouse gases are water vapour, carbon dioxide, methane, nitrous oxide and ozone.

The reason greenhouse gases are worrying is that the blanket of greenhouse gases around the planet is getting thicker, making the earth warmer. This phenomenon is known as 'global warming' or 'climate change' and is probably one of the most disastrous things to happen to the earth. It's happening because humans are releasing huge quantities of greenhouse gases into the atmosphere.

When life first evolved on earth, the greenhouse effect was a positive thing. It kept the earth warm enough for life to grow and thrive. Otherwise the earth would have been a cold planet, like the moon. But what this has meant is that life evolved within a very narrow temperature range, and all plants and animals have specific temperature ranges beyond which they will die out. Not only that, local climates all over the world are dependent on global temperatures. And when global temperatures change, local weather patterns change too, in ways that cannot be predicted. As the earth gets warmer, oceans currents change direction, monsoons become irregular, glaciers melt, reducing our supply of fresh water, and sea levels rise, drowning coastal cities.

Let us look more closely at some of these gases. Water vapour is the biggest cause of the greenhouse effect, but it is relatively stable and its quantities have not changed much. Water vapour concentrations fluctuate from place to place, but human activity does not directly affect water vapour globally.

Some greenhouse gases are part of vital and basic lifestyles -- meaning they are necessities, not luxuries -- and not much can be done about them. For example, keeping cattle and sheep for milk, meat or wool causes greater quantities of methane in the atmosphere, as does paddy rice farming. But this is how most of Asia lives, and stopping rice cultivation and cattle rearing would mean having to find new sources of food and income.

The biggest greenhouse gas culprit, brought on by human activity, is carbon dioxide. Carbon dioxide (CO2) comes from a wide variety of sources, but mostly from burning organic matter. Anything that is powered by fuel such as petrol, diesel, coal or even wood, releases CO2. The more we depend on our cars, planes and trains, the more we release CO2 into the atmosphere. Our industries are powered by these fuels, as is agriculture that uses petroleum to power machinery, produce pesticides and fertiliser, and transport produce to market.

Electricity-generating plants powered by coal or oil, as are many in India, also release CO2. The world is now what is called a 'carbon economy', meaning that our worldwide economic system is based on burning petroleum or coal -- known as 'fossil fuels' -- and there seems to be no way of getting around this. Our transport, industry, food, heating and cooling, household electricity that powers all our appliances -- all are based on deriving power from these sources.

The one way to reduce CO2 in the atmosphere is to allow plants to absorb it. Plants use atmospheric CO2 to build their bodies, they way we use food. This means that trees can store large amounts of CO2 as they grow. Unfortunately, however, we are not planting enough trees, and are cutting down whatever trees there already are. The more forests we destroy, the more CO2 we release into the atmosphere, compounding the problem caused by our use of fossil fuels. And it is the most industrialised and developed countries that create the most greenhouse gases, because they are the ones that burn the most fossil fuels, with their heavy use of cars and other energy-intensive devices. These are 'luxury' emissions of CO2, not essential ones, since people can do without their cars and planes. Developed countries also cut down the most forests (in their own country or in others) to fuel their industrial growth.

Most scientists feel that it is now too late to reverse climate change. But we can slow it down, hoping that we will be able to adapt to the changes already brought about by global warming.

The only way to slow down the process is to drastically reduce our production of greenhouse gases. Right now, it seems as if the only way to do this is to reduce our dependence on fossil fuels and shift to more efficient industries, transportation and lifestyles. An international agreement has been put in place to do just that: the Kyoto Protocol demands a reduction in the amount of various greenhouse gases produced by each country. But, although change is happening, it's happening very slowly and we may have already crossed the point of no return. In any case, the US, the world's biggest producer of greenhouse gases, has refused to sign the Kyoto Protocol, so whether other countries can have the desired impact is questionable.